These leaps between December and January of a year using the yyyymm format are the reason why a xy-plot looks like a stair. To get the correct x-axis you have to set some additional resources (tmXBMode = "Explicit", tmXBValues, tmXBMinorValues, tmXBLabels) which are used for the lower plot in the panel.

;--------------------------------------------------------------------;-- second plot;--------------------------------------------------------------------;-- generate time index and years arrays for explicit x-axis use inttime = ispan(1,ntime,1) ;-- create an index array for x-axis dimension time years = str_get_cols(tostring(time),0,3) ;-- retrieve the years